1. Field of the Invention
The present invention relates to an apparatus and method for providing relief to a hydraulic choke from exposure to excessive backpressure. More particularly, the present invention relates to pressure reducing valves and valve openings having shearable restraints.
2. Description of the Related Art
The present invention is applicable to hydraulic choke valves, which are a subclass of pressure reducing valves. Choke type pressure reducing valves generally function by causing a portion of the potential energy of a pressurized fluid to be dissipated through turbulence when the pressurized fluid is passed through a restrictive orifice. Typically, the orifice of a choke valve is selectably variable through reciprocation of its valving member toward and away from the valve seat, so that a desired combination of flow and exit pressure may be obtained.
A choke valve is normally open and is designed for one-way flow. This construction differs from that of a relief valve, which is another type of one-way flow valve that is normally closed. The present invention is applicable to a choke valve that differs in construction from the most common arrangement of such valves in having its flow gate pressure-balanced. Because of the pressure balancing of the sealing plug, the actuating loads on the stem of the flow gate are considerably reduced compared to those of most choke valves of comparable capacity. However, the stem of the flow gate, which connects to the reciprocable control screw of the actuator used to reciprocate the flow gate for this type of choke, is not pressure balanced. The present invention has as its purpose the release of excessive backpressure induced axial loads on the flow gate stem in order to avoid overload of the stem or the actuator that is attached to the stem.
The concept of a shearable release as a means of opening a relief valve on a one-time basis prior to rebuilding of the valve is disclosed in Allen U.S. Pat. No. 2,304,491, where a common nail is used as a shear pin. The sealing plug of the Allen valve is directly restrained against reciprocably unseating by the nail.
West U.S. Pat. No. 4,587,987 discloses a relief valve very similar to that of Allen, but with an indirectly acting shear mechanism. West uses a four-bar linkage mechanism that has a shearable link interconnecting two of its arms. The shaft of the West sealing plug is restrained by abutting one bar of the linkage, where one end of that bar is held by the shearable link. When the link shears due to excessive reaction loads to the forces applied to the supporting bar by the valve plug, the plug unseats to release the pressure.
Risinger U.S. Pat. No. 4,359,094 discloses a relief valve that has multiple directly acting releases for its sealing valving members, wherein the valving members are held by shear screws or shear pins. This particular construction is made to be inserted into a well bore to control a flow bypass.
While the references above do show shear release means for opening relief valves that are closed, there is a need for a shear release means for pressure reducing valves that are either open or closed.